Revise the pattern!
I designed one almost exactly like it and decided it was far too heavy for my taste. And like you, I decided it had too much filling anyway. So I redesigned it so it would take less filling, with the added benefit that the filling stayed distributed more evenly. I would post a pic, but I gave it to my mother and haven't made another one yet. I'll see if she can send me a pic I can post. Until then, I'll try to describe what I did.
The first thing I did was trim off and round out the corners on both the back and front, but particularly the back. I cut a lot off the back corners! I discovered it didn't make a bit of difference in the effectiveness.
The next thing I did was add what I called vertical seams. In the original version, there are only horizontal seams. On the back channels, I put in 3 vertical seams so there are 4 little pockets per horizontal row. I put an outside seam down the middle. Then I added about a quarter to a third cup of filling, then sewed an outside seam halfway to the edge to lock in that filling, which also created the next pocket. Put in the filling and sew the edge shut. Then do the other half. Do the same until you have all the filling in. Don't be afraid to shorten the width, either. Like I said, it will still be very effective. Use your judgment on how much filling to use. You DON'T have to put nearly as much as you think. Don't try to fill the pockets completely. It's just not necessary, and yes, it still works wonderfully.
I fill mine with a mixture of deer corn and rice. The deer corn holds in heat a little longer but the rice tends to mold itself better to your curves. I've used deer corn, rice, all kinds of dried beans, dried lentils, dried peas, and such, all with success. Sometimes it's a matter of what's on sale and what I have on hand!
update... okay, I found the pieces I had cut out for the next one I'll make. They're roughly 21 inches across by 24 in length. Here's a pic. It'll be much smaller than that once the filling is in.
Good luck!
Terri